1. Field of the Invention
The present invention relates to a technology for producing oil quench hardening and tempering and hard drawn steel wire (oil tempered hard drawn steel wire) having a shaped cross section. More particularly, it relates to oil tempered and hard drawn steel wire which can be easily coiled for the production of springs having superior characteristics, and also to a process for producing the same.
2. Description of the Prior Art
The wire used to make oil springs usually has a round cross section; however, one having an oval cross section, trapezoidal cross section, or any other shaped cross section is desirable for springs used under severe conditions. Although recent coil springs need to meet stringent requirements and to have high quality, it is impossible to make such coil springs from conventional wire having a round cross section. Consequently, studies are being made on oil tempered steel wire having a shaped cross section, and it has been put to practical use in some special fields.
Oil tempered round steel wire is usually produced from hot rolled wire rod by the steps of pickling, annealing, pickling again, cold working (to form the round cross section), and oil tempering. It is noted that the cold working such as wire drawing to make the shaped cross section is performed between the steps of the pickling (descaling) after annealing and the oil tempering.
A disadvantage of this process is that the steel wire is slightly twisted after oil tempering because of unbalanced cold working stress and quenching stress. Such twisted steel wire cannot be coiled into a spring having a normal form. Forced coiling with a jig breaks the wire. Springs made from twisted steel wire are subject to uneven stress distribution, insufficient compressive deflection, and early fatigue failure. They do not meet the design requirements, and yet they are poor in quality and yields.
Another disadvantage of oil tempered steel wire is that it is subjected to more surface flaws than round steel wire because it is coiled while it still has scale formed by heat treatment. In addition, it needs a coiling jig that requires complicated maintenance. For reasons mentioned above, oil tempered steel wire having a shaped cross section is used only in special fields.